The Mental Health Clinical Research Center (MHCRC) is dedicated to investigating the biological basis of psychopathology. A major goal of the center is to study candidate biological markers of mental illnesses with particular attention paid to questions of disease specificity. Issues of disease specificity are critical to the validation of current nosological criteria and mechanistic hypotheses of disease pathogenesis. Several investigators with independent laboratories participate in the center. Center investigations include studies of: 1) cerebrospinal fluid and serum concentrations of neurotransmitters, neurotransmitter metabolites, and psychotropic medications; 2) highly specific pharmacological treatments to probe target neurotransmitter systems; 3) brain structure, particularly by CT and MR mapping; 4) sleep electrophysiology; 5) waking psychophysiology (i.e., event-related potentials); and 6) biostatistical methodology. Patients are admitted to the MHCRC and maintained medication-free for most protocols; parallel non-patient control data are also collected. Diagnosis, ongoing clinical assessment, treatment, statistical support, database maintenance, and quality control are core center functions. A large cross-sectional database on patients and controls has been established and is a significant resource.